In a number of applications having special requirements, it is necessary to substitute, in place of uniform materials, specific composite materials. These composites are often formed by joining components made from different materials. Oftentimes, the individual components display widely differing properties.
For example, composite materials formed from graphite and high-melting metals such as molybdenum or tungsten have proven suitable for special high-temperature applications. As compared with one-component materials formed from high-melting metals, these composites allow a wider range of high-temperature applications, especially because of the improved heat storage capacity and because of the lower density of graphite.
One important application readily availing itself to the advantage of such a composite material includes, for example, rotating anodes for X-ray tubes.
Other metal/ceramic composite materials are also of technical interest and are increasingly used. Composite materials of ceramics such as Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, SiC, TiO, TiN, TiC, Zr).sub.2 or AlN, with high-melting or other metals, may be mentioned as examples in this context.
Furthermore, ceramic/ceramic composite materials, formed by joining individual ceramic components having different properties, are also used for special applications.
Examples of products for which such composite materials are used include, without limitation, rotating anodes for X-ray tubes; first wall components, diverter plates, and limiters for fusion reactors; high tension switches; continuous casting molds; electrodes; and like applications.
The decisive feature for the usefulness of all these metal/ceramic and ceramic/ceramic composite materials is a mechanically good joint between the individual material components. In many cases, the joint must also be resistant to high temperatures. A brazing operation is often used to join the individual, different materials to one another, as this also ensures good thermal conductivity between the individual materials. Brazing materials such as Zr, AgCuTi, CuTi and NiTi have gained acceptance, especially for high-temperature applications.
A problem often encountered with these composite materials is that states of stress frequently arise, owing to the widely different coefficients of thermal expansion frequently displayed between the composite components to be joined. These stress states can impair the strength of the brazed joint and, in extreme cases, can cause destruction of the component.
In order to improve the strength of the brazed joint, it is conventional to increase the brazing surface area of the ceramic material. This can be achieved, for example, by introducing grooves or striae onto the brazing surfaces by special mechanical treatment, such as by means of a conventional turning or milling operation.
In German Auslegeschrift 1,951,383, which relates to a composite rotating X-ray tube anode of a high-melting metal part and one or more graphite parts fitted therein, it is proposed to provide the brazing surface of the graphite parts with certain structurings, such as grooves, which increase the surface area. However, this known structuring of the brazing surface is frequently insufficient to obtain satisfactory strength of the brazing joint.
Austrian Patent 362,459 describes a method of joining the individual parts of a rotating anode, composed of a high-melting metal with one or more graphite parts, by brazing. In this method, slots are introduced into the graphite surfaces which are to be brazed. The slots merge into bores which are perpendicular to the brazing surface and which penetrate the graphite parts. The bores penetrating the graphite parts allow the gases formed during brazing to escape to the outside, thereby largely avoiding the formation of cavities in the brazing material layer.
However, in practice this method does not result in a strength-increasing enlarged brazing surface, due to the small number of the bores employed and to their physical dimensions. The bores merely serve to discharge gases formed during the brazing operation. Therefore, even in the brazed joints produced by this method, the strength of the joint between the different materials frequently does not meet the stringent requirements for which the materials are intended in use, especially with respect to thermal shock resistance.
German Offenlegungsschrift 2,759,148 describes a method of making a brazed joint between parts of pyrolytic graphite, with one another or with a metallic part. The part consisting of pyrolytic graphite is provided with a recess into which the brazing material flows during the brazing step, thus filling the recess. The method is matched specifically to produce joints used in the construction of electronic tubes, where the parts to be joined are very thin, having thicknesses of between 20 and 100 .mu.m. In the described embodiments, the recesses are exclusively made as bores or slots which completely penetrate the pyrolytic graphite part. This leads to a brazed joint in which the brazing material frequently flows off from the contacting surfaces of the composite components, and the brazing material escapes via the bore on the surfaces which are not in contact. The result is a punctiform joint resembling a riveted joint. Such a joint is not suitable for joining solid parts of large volume.
German Offenlegungsschrift 1,652,840 describes a method in which a metallic part and a ceramic part are joined by brazing. The brazing surface of the metal part is provided with capillary-active flow channels for the brazing material, preferably in the form of parallel striae. They may also be formed by a knurling operation. The disadvantage here is that, in many cases, even increasing the surface roughness in this way is not adequate to obtain the optimum strength of the brazed joint.